1. Technical Field
The present invention relates to a half-wave plate made in particular of an inorganic crystalline material having a birefringent property and rotatory power such as quartz crystal. Further, the invention relates to an optical pickup device, a polarization conversion element, and a projection display device using the half-wave plate.
2. Related Art
In the past, half-wave plates for outputting outgoing light, which is linearly polarized light having a polarization plane obtained by rotating the polarization plane of linearly polarized light as the incident light by predetermined angle, for example, 90°, have been used in optical devices such as optical pickup devices used for recording/reproduction in optical disk drives, polarization conversion elements, or projection display devices such as liquid crystal projectors. In general, as a material of the wave plate, there are used a resin film made of an organic material such as polycarbonate provided with a birefringent property by a stretching process, a retardation plate obtained by sandwiching a polymer liquid crystal layer with transparent substrates, and a crystal plate made of an inorganic material having a birefringent property such as quartz crystal (see, e.g., JP-A-2005-208588, JP-A-2006-40343, JP-B-52-4948, and JP-B-3-58081).
In particular, optical pickup devices used for recording/reproduction in optical disk drives each adopt a blue-violet laser with an extremely short wavelength and high power in order for achieving high density and high capacity recording. However, the resin films and the liquid crystal materials made of the organic materials described above have properties of being apt to absorb light in a range of blue through ultraviolet. Therefore, the materials absorb the blue-violet laser to generate heat, and might cause deterioration of the materials themselves to hinder the function of the wave plate.
Further, in optical disc recording/reproduction devices compliant with the Blu-ray standard, the half-wave plate is disposed adjacent to the laser source of each of the optical pickup devices, and is exposed in an extremely high temperature environment for a long period of time. Similarly, in liquid crystal projectors, the half-wave plate is disposed the closest to the white light source, and is exposed in an extremely high temperature environment for a long period of time. Therefore, in either case, the half-wave plate is required to have high light resistance and long-term reliability. In this viewpoint, the inorganic crystal materials such as quartz crystal have extremely high light resistance, and in particular, quartz crystal wave plates are advantageously used in the optical systems using the blue-violet lasers.
Further, the quartz crystal has not only the birefringent property but also the rotatory power with respect to the direction of a crystal optical axis. It is well known that the rotatory power can have an influence on the performance of the wave plate depending on the cutting angle of the quartz crystal plate. In order for eliminating the influence of the rotatory power, there is proposed a wave plate having two wave plates made of an optical material stacked so as to overlap with each other with the respective optical axes intersect with each other, and arranged so that the phase difference between the both wave plates, optical axis azimuth angle, rotatory power, and the angle formed between the rotation axis and the neutral axis satisfy a predetermined relational expression, thereby improving characteristics in a broad spectrum (see, e.g., JP-A-2005-158121).
However, in the half-wave plate formed of a quartz crystal single plate, the design method considering the variation in the polarization state due to the rotation of the polarization plane caused by the rotatory power in addition to the variation in the polarization state due to the phase difference caused by the birefringent property has not ever been reported as far as at least the inventors know. Therefore, there arises a problem that if the design phase difference of the quartz crystal half-wave plate is set to 180°, the conversion efficiency does not necessarily become 100% depending on the cutting angle of the quartz crystal plate.
The polarization state of the quartz crystal half-wave plate will be explained using the Poincare sphere shown in FIG. 11. Assuming the reference point of the incident light of the linear polarized light as the point P0=(1, 0, 0) on the S1 axis, the rotational axis R1 is set in a position specified by rotating the S1 axis as much as the angle 2θ1 (θ denotes the optical axis azimuth angle of the quartz crystal plate), and further tilting it toward the north pole (S3) as much as the angle 2ρ (ρ denotes the rotatory angle of the quartz crystal plate) with respect to the S1-S2 plane. When rotating the point P0 rightward as much as the phase difference Γ=180° around the rotational axis R1, the resulting point P1 corresponds to the position of the outgoing light.
In FIG. 11, in the case of the optical axis azimuth angle θ1=45°, since the rotational axis R1 exists on the S2-S3 plane, when rotating the point P0 rightward around the rotational axis R1 as much as 180°, the position of the outgoing light always comes to the point P1=(−1, 0, 0) on the equator of the Poincare sphere. On this occasion, the conversion efficiency of the quartz crystal wave plate is 1. However, in the case of the optical axis azimuth angle θ2≠45°, since the rotational axis R2 fails to come on the S2-S3 plane, a point P2 shifted from the equator of the Poincare sphere becomes the position of the outgoing light. Therefore, the conversion efficiency of the quartz crystal wave plate is degraded.
The rotatory power of the quartz crystal plate becomes the strongest in the optical axis direction of the quartz crystal. Therefore, the larger the cutting angle of the quartz crystal plate, namely the angle formed between the optical axis thereof and the normal line with respect to the principal surface thereof, is set, the smaller the influence of the rotatory power becomes. FIG. 12 shows the result of the simulation of the variation in the conversion efficiency T with respect to the cutting angle for respective values of the optical axis azimuth angle θ (θ=5°, 10°, 15°, 20°, 25°, 30°, 35°, 40°, and 45°) in the quartz crystal half-wave plate with the design phase difference of 180°. According to the drawing, it is understood that the conversion efficiency is remarkably degraded with the cutting angle equal to or smaller than 30°.
Therefore, by setting the cutting angle to be larger than 30°, the high conversion efficiency approximating 1 can be obtained. However, in the range of 30 through 90°, the thickness of the quartz crystal plate becomes as thin as about 10 through 26 μm. Therefore, since the strength of the quartz crystal plate is remarkably degraded to make the quartz crystal plate easy to be broken, handling in manufacturing processes and in use is extremely difficult.
Further, the blue-violet lasers cause a problem that wavelength of the oscillated laser drifts (varies) when generating high heat and expanding in use. Therefore, in the case of using the blue-violet lasers in the light sources of the optical pickup devices, there arises in the half-wave plate a problem that the conversion efficiency of the linearly polarized light is deteriorated due to the wavelength drift of the incident laser beam. In particular, since the half-wave plate in a higher-order mode has a large thickness, an amount of the variation thereof increases as the phase difference increases, and the conversion efficiency is more significantly deteriorated.